Description of the surface and subsurface material conditions after applying high frequency mechanical impact (HFMI) treatment, shot peening and deep rolling

The advantages of removing the potential threats of unwanted residual stresses and exploiting the beneficial residual stresses by thermal and mechanical treatments are already known in welding communities. Beside the fact that some of the recently developed mechanical surface treatment techniques in other industrial disciplines e.g. deep rolling have not been applied for welds yet, a shortcoming in general is that a quantitative integration of these advantages in the fatigue design has not kept pace with the developments in the field of characterization of polycrystalline materials. Since the fatigue cracks almost always start from the surface, the characterization of the surface zone after applying these fatigue improvement techniques is of paramount importance. In this paper the influence of cold working on inducing the compressive residual stresses in the surface zone after high frequency mechanical impact treatment (HFMI), shot peening and deep rolling has been studied by means of hardness measurements and synchrotron diffraction techniques. How these processes influence the topography and the geometry of the notches has been studied by scanning electron microscopy.